Spinal Muscular Atrophies are collectively the second most common autosomal recessive neurodegenerative group of disorders with an incidence of ˜1 in 6000 (Crawford, T. O. and Pardo, C. A., 1996) and a carrier frequency of ˜1 in 35 (Feldkotter, M. et al., 2002). The diseases are caused by the loss of α-motor neurons resulting in subsequent atrophy of voluntary muscle groups leading to paralysis and eventually to premature infantile death. Genetically the types of SMA result from a homozygous loss or mutation in the telomeric copy of the Survival Motor Neuron-1 (SMN1) gene. All SMA patients rely on the nearly identical copy gene, SMN2, which produces low levels of functional SMN protein. SMN is ubiquitously expressed and is a critical factor in a variety of RNA pathways. The best characterized SMN activity is in the assembly and maturation of the spliceosomal UsnRNPs (Meister, G., et al., 2002; Pellizzoni, L., et al., 2002). Even though the SMN2 gene is 99% identical in nucleotide sequence and is completely identical in amino acid sequence, approximately 90% of SMN2-derived transcripts are alternatively spliced and encode a truncated protein lacking the final coding exon (exon 7). This aberrant splicing event is the result of a silent, non-polymorphic C to T nucleotide transition 6 nucleotides within exon 7 (Lorson, C. L., et al., 1999; Monani, U. R., et al., 1999). SMN2, however, is an excellent target for therapeutic intervention.
Cis-acting negative regulatory regions that surround SMN2 exon 7 have been identified and described (Lorson, C. L., et al., 1999; Miyaso, H., et al., 2003; Miyajima, H., et al., 2002). In particular, ISS-N1 has been a hotspot for experimental therapeutics, especially antisense oligonucleotides (ASOs). ASO molecules of various lengths and backbone chemistries have been used to inhibit the repressor activity of ISS-N1, leading to an increase in SMN protein and significant extensions in survival in animal models of SMA. For example, one such approach is described in U.S. Pat. No. 8,110,560 B2 to Singh et al., which discloses a series of oligonucleotide reagents that effectively target the SMN2 ISS-NI site in the SMN2 pre-mRNA. U.S. Pat. No. 8,110,560 teaches that the ISS-N 1 blocking agents target the SMN2 pre-mRNA to modulate the splicing of SMN2 to include exon 7. 2′-MOE chemistry has been used by ISIS Pharmaceuticals in the development of their ASO, SMN-Rx (Hua, Y., et al., 2011; Rigo, F., et al., 2012). Similar Morpholino-based ASOs have shown excellent pre-clinical promise in severe SMA mice and are under further development.
Still, no effective treatment exists for SMA, and the complexity and expansive clinical spectrum suggests that the SMA community cannot solely rely upon a single lead compound or genetic target.